Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.
Usually natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stock suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled (to reduce flashed vapour through one or more expansion stages) to final atmospheric pressure suitable for storage and transportation. The flashed vapour from each expansion stage can be used as a source of plant fuel gas.
In LNG plants, streams comprising a mixture of vapour and liquid phases pass, for example between two heat exchangers. One example is shown in FIG. 3 of U.S. Pat. No. 6,389,844 B1.
U.S. Pat. No. 6,389,844 B1 relates to a plant for liquefying natural gas. FIG. 3 shows an embodiment for pre-cooling the natural gas, involving first and second stage heat exchangers 102′ and 102. Between these first and second stage heat exchangers, there are two conduits 150 and 151, one for refrigerant and one for natural gas. The refrigerant and natural gas are mixed vapour and liquid streams, and such streams are carried by a single conduit between the heat exchangers 102′ and 102.
However, this way of passing streams between two heat exchangers can result in an uneven distribution of the vapour and liquid phases of the streams passing through the conduits 150 and 151. As a consequence, there may be non-uniform distribution of the vapour and liquid phases going into the second stage heat exchanger 102, which results in an uneven temperature distribution and therefore inefficiency in the second stage heat exchanger 102.
It is an object of the present invention to improve passing a mixed vapour and liquid stream between two heat exchangers.
It is a further object of the present invention to reduce the energy requirements of a cooling plant or method.